This invention relates to optical glass having a refractive index (n.sub.d) equal to or greater than 1.60 and an Abbe number (v.sub.d) equal to or greater than 58.
The most relevant reference describing the state of the art in respect of this invention is U.S. Pat. No. 4,343,908, issued Aug. 10, 1982, incorporated by reference herein, along with all references cited therein.
According to this reference, glasses of the same optical type can be manufactured with higher chemical stability; however such glasses, particularly at high Abbe numbers (.gtoreq.60) show a strong tendency to phase separation, as has been known for a long time in the SiO.sub.2 /B.sub.2 O.sub.3 system. (See W. Vogel, Glaschemie, VEB Deutscher Verlag fur Grundstoffind, Leipzig 1979, page 152). This phase separation tendency is exacerbated by the addition of fluorine which is added for the purpose of lowering dispersion at high refractive indices (.gtoreq.1.620), thereby permitting Abbe numbers of over 60.0.
Another disadvantage of the glass of U.S. Pat. No. 4,343,908 is that application of a fine-cooling treatment (10-0.3.degree. C./h) necessary for optical glasses is no longer possible without producing clearly preceptible clouding in larger glass blocks (.gtoreq.1000 cm.sup.3), the clouding limit extending down to the transformation temperature. Thus, within the composition ranges of U.S. Pat. No. 4,343,908, clouding cannot be prevented in the high refractive index/Abbe range.